Group:SMART:P-glycoprotein: Why Cancer Drugs Fail

Abraham Lincoln High School, San Francisco CA, SMART Team


Team members: Tiffany Cai, Pauline Chan, Maggie Chang, Tina Chen, Desiree Chiu, Jeffrey Chum, Jane Hwang, Nancy Kong, Grace Wang

"Poster Team:" Tiffany Cai, Tina Chen

"Model Team:" Jane Hwang, Nancy Kong, Grace Wang

"Abstract Team:" Pauline Chan, Desiree Chiu

"What is P-gp Team:" Maggie Chang, Jeffrey Chum

Teacher: Mr. Richard Gin

UCSF: Anita Grover (mentor), Ben Koo, Kurt Giles, Sabine Jeske

Abstract
P-gp is an efflux drug transporter located in the cell membrane of the cell. It transports foreign and unfamiliar substances out of the cell, protecting the body from harmful toxins that stay in the bloodstream. An example would be cleansing toxins from the small intestine when consuming food that may have gone bad. In this case, P-gp would be acting on behalf of our benefit. However, in cancerous cells, P-gp is the main factor that stops treatment from working against cancer. It causes 90% of chemotherapy to fail in patients. Although the drug may be good for the body, P-gp is unable to recognize the drug so it pushes it out of the cell once it enters. P-gp is why most cancer drugs fail to have any effect in the body. Recently the X-ray structure of P-gp was resolved (Aller et al, 2009). We have built a 3-D physical model based on this X-ray structure which highlights the hydrophobic and hydrophilic regions of the protein, nucleotide binding domains, and active site residues that are important for verapamil, a common heart medication, binding. As it has not yet been possible to identify whether a drug is or is not a substrate of P-glycoprotein, this 3-D model may lend insight into this prediction.

So... What is P-gp?
P-glycoprotein (P-gp) is an active transporter in the cell membrane of liver, blood-brain barrier and small intestine cells. P-gp’s role is to guard the body against foreign and harmful substances by transporting “toxins” out of the cells. P-gp is one of the body’s best line of defense against harmful toxins; it prevents damage to cells. Normally, P-gp would be considered a protein that is beneficial for the protection of our body, but in the case with pharmaceuticals, it is considered an obstacle. Drug companies have produced drugs that can possibly treat cancer; however, P-gp recognizes these drugs as harmful or toxic to the body. As a result, P-gp would eject the drug before it can take its desired effects on the body. Nevertheless, P-gp is crucial to the human body. For example, P-gp in liver cells help remove toxins in blood cells. The P-gp in blood-brain barrier cells protect the brain from dangerous substances in the blood while the P-gp in the small intestine help remove toxins from digested food.

Our Model
In our model of P-glycoprotein, the hydrophilic regions are highlighted in magenta, the hydrophobic regions are highlighted in violet , the active site residues for the drug Verapamil(-) is highlighted in green , and the Nucleotide Binding Domains or NBDs are highlighted in yellow. Since P-glycoprotein is a dimer, a molecule consisting of two similar subunits, only half of the dimer (or one of two subunits) is shown in our model.

Because P-glycoprotein is embedded in the cell membrane, it contains both hydrophilic and hydrophobic regions. If you look closely at the protein model, you will notice a band of violet or a hydrophobic band. The hydrophobic band is the region of the protein that is embedded in the cell membrane (since the inside of the cell membrane is hydrophobic and the outside of the cell membrane is hydrophilic). Both the hydrophobic and hydrophilic regions are critical in P-glycoprotein’s function in transporting drugs out of the cell. The active site residues that are important for the transport of Verapamil are highlighted with the color green. These residues are H60, A63, L64, S218, I302, L335, A338, F724, I864, G868, F938, T941, L971, V978, G980, and A981. The Nucleotide Binding Domains (NBDs), which are highlighted in yellow, are important because after binding to ATP, the conformation of the protein changes. By changing the conformation of the protein, drugs are transported out of the cell. P-glycoprotein is also an active transporter because it requires energy of ATP in order to transports drugs out of cells.

Not shown on the model are flaps(--) that aid in guiding drugs into the cavity of the protein. Once the drug reaches the cavity, it would bind to specific residues (that are similar to those Verapamil residues (in green). Once ATP binds to the NBDs of the protein, the protein would change its conformation, taking the drugs out. The original conformation of the protein is an upside down V. When ATP binds to the NBDs, the NBDs close up, and the tip of the protein opens up, changing the conformation of the protein in the shape of a V.

- For more information about Verapamil, please refer to ‘Medical Implications of P-glycoprotein.’

-- The flaps are not shown because they are too fragile to crystallize.

Medical Implications
The function of P-glycoprotein (P-gp) provides the tumor cells of cancer patients with resistance to a variety of anti-cancer drugs such as Vincristine because it pumps out any drugs that enter the tumor cells. Due to such activities, the dosage of drugs that enter the cell is inadequate in fulfilling its duty and taking its desired effect.

Depending on the dosage, Verapamil can be a P-gp inhibitor. For example, Verapamil can help anti-cancer drugs enter targeted cells by “distracting” P-gp; thus, decreasing P-gp’s activity on anti-cancer drugs. When Verapamil is taken with Vincristine, an anti-drug, the effects are beneficial to the patient because P-gp would be focused on transporting Verapamil out, allowing for Vincristine to enter the cell. This offers hope and promise in increasing the effectiveness of chemotherapy in cancer patient though the use of anti-cancer drugs with P-glycoprotein inhibitors in does determined for each individual.

Verapamil, a P-gp inhibitor, helps anti-cancer drugs go into cells by “distracting” P-gp; thus, decreasing P-gp’s activity on anti-cancer drugs. For example, when an anticancer drug such as, Vincristine, is taken with Verapamil, the effects are beneficial to the patient because P-gp is focused on taking Verapamil so that Vincristine can get into the cell. This offers promise in increasing the effectiveness of chemotherapy in patients with cancer through the combined use of anticancer drugs with P-glycoprotein inhibitors (in doses determined for each individual).

Additional Resources
For additional information, see: Cancer